Inorganic salts, such as sodium chloride or sodium sulfate or the like, can be formed as secondary products in the production of anionic, cationic or amphoteric surfactants and also nonionic surfactants. These inorganic salts can be troublesome in certain special applications, for example when any corrosion-promoting potential of the surfactant solution is to be strictly avoided or when salts influence the viscosity of the surfactant solution.
However, the removal of inorganic salts from organic acids or salts is difficult. For laboratory-scale and pilot-plant quantities, processes based on column chromatography may be applied or organic solvents may be used for precipitation. However, both processes are extremely complicated and do not always produce the required result.
Now, the problem addressed by the present invention was to provide an industrially workable process which would enable solutions of organic salts or acids capable of micelle formation to be freed from inorganic salts and also from other impurities incapable of micelle formation. In solving this problem, the invention makes use of the knowledge that, both in the ultrafiltration and in the reverse osmosis of solutions containing inorganic salts and organic salts or acids of the type mentioned, the inorganic salts pass over into the permeate while the organic salts or acids capable of micelle formation remain in the retentate. This is surprising insofar as the molecular weight of the organic salts or acids capable of micelle formation is lower than the cutoff molecular weight of the corresponding membranes, so that the expert had to assume that separation would not be possible in this case.
The surprising separation effect cannot be scientifically explained. However, it is assumed that secondary membrane formation and/or micelle formation are jointly responsible.